CertVerifier: Prevent Chrome from crashing on public slot for kiosk

Chrome is intentionally crashing when it fails to load the public
slot because it might contain trust settings for CA certs. Kiosk
sessions don't have the UI to change trust settings or use the
public slot in any other way, so CertVerifier doesn’t have to load it.

This is a followup from https://crrev.com/c/5349523 .
Another place in Ash already initializes NSS with the internal
slot for kiosk. Depending on which place is executed first
(not deterministic), Ash will use different slots, which should
not be the case.

As a note, Lacros should also use the internal slot for kiosk,
but that will be changed in a separate CL.

Bug: b/256076122
Test: Existing automated tests
Change-Id: I1f4b1782f8420324c26378069e3e673757cc6d94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5544183


Commit-Queue: Michael Ershov <miersh@google.com>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1304294}